Extract StaticConfigTrait.

Extract a trait for the config(),drop(),configured() duck type used in
a number of classes. Implement the trait in Cache. This also removes the
Closure factory function. It will not be usuable by Email, and I think
it provides questionable value right now. If it becomes useful in the
future it can always be re-added.
Update Log to use StaticConfigTrait.

Update Log to use the new trait. This simplifies some of the internals
of Log as _init() is not called quite as often but still gives the same
results as before.
Add additional test assertion for Log::drop()
Update Email to use the StaticConfigTrait.

Fix an issue in the trait where errors would be generated by
implementing classes lacking a $_registry property.
Update ConnectionManager to use StaticConfigTrait.

This removes support for the datasource key which isn't necessarily
a bad thing. Update drop() to be conformant with other implementations
in CakePHP.
Remove package tag.
Remove Closure version and update docs.
Inline method that doesn't need to exist.
Add missing test to show that profile() throws exceptions

When using an invalid profile name an exception should be raised.
Merge pull request #1559 from markstory/3.0-config-trait

3.0 extract config() methods into a trait